One female spotted hyena in the Ngorongoro Crater died in 1996. Twenty-seven years later, every hyena born into her clan and still alive was her descendant.
Almost half the females in that same crater never had a single grandcub.
Count only one generation of cubs, and male hyenas appear to be the more unequal sex, exactly as biologists expect. Count grandcubs instead, and the females take that title.
Marta Mosna and colleagues at the Leibniz Institute for Zoo and Wildlife Research in Berlin followed all eight hyena clans in the crater for 29 years.
Field teams tell every hyena apart by its spot pattern and the notches in its ears. Fathers are harder to know by sight, so the team read DNA from tissue, hair, and the mucus scraped off fresh droppings.
In the end, the researchers pieced together 2,743 hyenas with a known mother, eight generations deep.
Cubs counted one year at a time
Sperm are cheap. Eggs are not, and a female mammal pays again in pregnancy and milk.
Males gain more from mating often, so a few of them father a great many cubs while most father none.
Biologists call that reasoning the Darwin-Bateman paradigm. Under it, males should be the more unequal sex.
Janicke and colleagues tested that across the animal kingdom in 2016 and found the same difference in most species.
Mosna’s team followed 287 females and 205 males from the start of their breeding lives to the end. The team measured how evenly each sex produced cubs.
Zero on that scale means every animal breeds at the same rate.
Females scored 0.19 and males 0.41. Males came out the more unequal sex, exactly as predicted.
Over a whole lifetime, though, the sexes came out level. The top fifth of each produced half of all the cubs: 19.5 percent of females, 19.8 percent of males.
A few mothers dominate the family tree
Next, the team counted the next generation, and the one after that.
Just 8.2 percent of the females produced half of all the grandcubs. Among males, 12.4 percent did. One generation later, 6 percent of females and 9.4 percent of males produced half of the great-grandcubs.
Forty-seven percent of the females left no grandcubs at all, against 33.7 percent of the males. By the next generation, 56.5 percent of females and 43.1 percent of males had none.
“Reproductive inequality among females increased with every generation we included. Privileged lineages didn’t just remain successful — they grew ever larger,” said Eve Davidian, who co-directs the Ngorongoro Hyena Project and works at the Institut des Sciences de l’Évolution de Montpellier.
Among hyenas that produced 10 or more cubs, the females with the most cubs also tended to have the most grandcubs. Males with many cubs often had few.
Most of those females had been born near the top: 71.4 percent had a high-ranking mother.
Daughters keep rank, sons lose it
Every clan keeps a strict order of rank, with one alpha female above everyone. Cubs enter that order just below their mother. Daughters hold their place for life, and sons lose it the day they leave home to breed.
Philemon Naman, a field assistant with the project in Tanzania, explained: “A high rank confers advantages on both sexes in the early stages of life, such as priority access to food, faster growth and a higher survival rate.”
About 85 percent of males leave the clan they were born in before they breed. Females almost never do. An immigrant male joins his new clan at the bottom and waits his turn there.
A male born to a high-ranking mother keeps his priority at a carcass on visits back to his birth territory. He loses it about two years after he leaves. His sisters never lose any of it.
High-ranking females start breeding earlier, wait less time between litters, raise more cubs to adulthood, and live longer than low-ranking ones.
Scrambling the family tree
Alpha females simply have more cubs, so of course their descendants outnumber everyone else’s. The team had to answer that objection.
To test the idea, the researchers scrambled the family tree. Each hyena received a randomly chosen mother from the same clan that had given birth within the previous two years.
High-ranking females remained more likely to be chosen because they produced more cubs. Only one connection was removed: the inheritance of social rank from mother to cub.
Then they ran the scramble a thousand times. Alpha-born hyenas still spread through the scrambled trees, but in none of the thousand runs did they spread as fast as they do in the real one.
About 9.5 percent of the hyenas had an alpha mother. Go one step further back, and 21.6 percent had an alpha grandmother.
Follow the same hyenas through their fathers instead, and far fewer of them turn up with an alpha in the line.
In each of the eight clans, the team picked out the female with the most living descendants, and all eight had held the alpha position at some point.
The female mentioned at the beginning of this story is M-003 of the Munge clan. On January 1, 2023, her 52 living descendants were the entire home-born clan.
A pattern beyond hyenas
Mosna’s team drew the great-grandcub figures from the smallest group in the study, 291 hyenas that began breeding in or before 2006.
When the team stopped counting, 150 of that group’s 1,284 cubs were still alive and still breeding.
No one can yet separate the effects of rank from everything else a mother passes down. In the team’s view, a high-ranking mother shapes almost every other condition her cub grows up in.
Biologists already knew two ways for females to end up the more unequal sex. Male seahorses carry the young. In meerkats, one dominant female does nearly all the breeding.
“This study identifies a third route to the same outcome: the inheritance of social privileges along the female line,” said Alexandre Courtiol, one of Mosna’s co-authors in Berlin.
Long-tailed macaques, yellow baboons, and vervet monkeys all inherit rank from their mothers. High-ranking macaque and baboon mothers produce more young over a lifetime, and high-ranking vervet mothers lose fewer infants.
No one has followed any of those species far enough to see whether the sexes swap places the way they do in the crater.
Anyone who tries will need what Ngorongoro has: every mother named, every father confirmed by DNA, eight generations deep. Only a handful of field teams anywhere keep records like that.
The full study was published in the journal Science Advances.
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